Machine tools are material cutting machines that are used in the manufacturing process for many different products. There are many types of machine tools, such as milling machines, lathes, and grinding machines. A milling machine is typically used to cut (i.e., mill) a desired shape into a raw piece of material (termed a workpiece). A milling machine typically comprises a movable table to which the workpiece is affixed. The table is typically capable of moving in two perpendicular directions, and the two different directions are typically termed the X-axis and the Y-axis. The table is typically connected to one or more devices (e.g., leadscrews) capable of translating the shaft rotation of one or more servo motors into the linear movement of the table. The movement of the table is therefore typically controlled by controlling the shaft rotation of the servo motors. The power supplied to each servo motor is typically regulated by corresponding servo amplifiers.
The table, and therefore the workpiece, is moved in a controlled manner relative to a cutting tool to enable the cutting tool to remove material from the workpiece to create the desired final product. The cutting tool typically attaches to a rotating shaft supported by rotational bearing, termed a spindle. The rotation of the spindle is driven by a spindle motor, with the power to the spindle motor regulated by a corresponding spindle amplifier. The spindle, along with the cutting tool, may also be moved relative to the workpiece to further control the removal of material from the workpiece. For example, the spindle may be moved up and down relative to the plane on which the machine tool sits. To enable the movement of the spindle relative to the workpiece, the spindle may be connected to a leadscrew which is in turn connected to a servo motor. This up and down direction is typically termed the Z-axis. While a typical three axis (i.e., X, Y, and Z) milling machine using servo motors and leadscrews is described above, many other configurations of milling machines exist. For example, milling machines may have five or more controlled axes. Additionally, milling machines may use electromagnetic linear drives, rather than servo motors and leadscrews, to move the table and the workpiece.
The rotation of all the servo motors are precisely controlled and coordinated to produce the desired movement of the workpiece relative to the cutting tool to create the desired finished shape. Additionally, the rotational speed of the spindle, and therefore the cutting tool, may also be controlled by controlling the rotational speed of the spindle motor. The servo and spindle motors and amplifiers are typically controlled by a special purpose controller, termed a computer numerical control (CNC). In addition to controlling the trajectory of the workpiece relative to the cutting tool, the CNC also controls the speed at which the workpiece is moved relative to the cutting tool. This speed is typically termed feedrate. The CNC is typically programmed to operate the machine tool at a specified feedrate desirably to utilize the machine capability without damaging the cutting tool or the spindle, or exceeding workpiece accuracy requirements.
The movement of the workpiece relative to the cutting tool as the workpiece is being milled creates both a tangential force and a radial force on the cutting tool. A torque is generated by the tangential force multiplied by the cutting tool radius and a bending moment (termed radial load) is generated by the radial force multiplied by the cutting tool length. The torque and radial load must typically be kept below a predefined maximum to prevent damage to the cutting tool and/or the spindle. The torque is typically monitored by monitoring the output power or current of the spindle amplifier. The radial load is typically monitored using strain gauges on the spindle structure. Circumstances may exist where the movement of the workpiece relative to the cutting tool at the programmed feedrate while the workpiece is being milled produces excessive torque and/or excessive radial loading. Adaptive control systems have been developed to react to the occurrence of such circumstances, such as the adaptive control system disclosed in commonly assigned U.S. Pat. No. 4,698,773 to Jeppsson, entitled Adaptive Feed Rate Override System for a Milling Machine, the contents of which are incorporated herein by reference in its entirety. Adaptive control systems typically repeatedly monitor the spindle power and the radial load as the workpiece is being milled. If the power and/or the radial load exceed a respective predefined maximum, the adaptive control system will typically cause the feedrate to be reduced to correspondingly decrease the spindle power and/or radial load. The adaptive control system may be a separate device capable of communicating with the CNC, or may be a functional element (e.g., hardware and/or software) within the CNC. The adaptive control system will typically cause this feedrate adjustment by modifying a feedrate override (FROV) parameter of the CNC. The FROV parameter is typically defined by a percentage, and the CNC typically is capable of using the FROV parameter to adjust the feedrate by that percentage. The change of the FROV parameter may be commanded by a machine operator and/or by the adaptive control system. For example, consider a machine tool in which the programmed feedrate is 20 inches per minute and the operator sets the FROV parameter to 100%. If the FROV is changed to 90% by the adaptive control system, the CNC will reduce the feedrate to 18 inches per minute. If both the power and radial load later drop below the predefined respective limits, the adaptive control system may attempt to increase the FROV back to 100% (or potentially to a FROV value greater than 100%) to increase the machining productivity.
The predefined maximum power and radial load may be determined based on, for example, the power capacity of the spindle and/or the spindle amplifier and the spindle radial load capability. The maximum power and radial load are typically loaded into parameter values in the adaptive control system at the beginning of an operation. These parameter values may be overwritten with new values as necessary. Known adaptive control systems typically use a single maximum value for each of spindle power and radial load, irrespective of the rotational speed of the spindle. However, the available spindle power and the spindle capability of sustaining the radial load both vary over the operating rotational speed range of the spindle. As such, known adaptive control systems may fail to fully utilize the spindle capacity by using maximum power and radial load values established for one spindle speed while operating at a different spindle speed.